The invention relates to two-dimensional spectrometers, wherein light to be analyzed enters an aperture and is subjected to cross-dispersion involving use of an echelle grating, and wherein a catadioptric camera images the aperture as a disperse light pattern at a focal plane.
Most optical spectrometers are single-dimensional in their presentation of spectral information. A prism spectrometer disperses light non-linearly as a function of the refractive index of the material of the prism. A grating spectrometer disperses light nearly linearly; the dispersion increases as the groove density increases, and the dispersion increases with the order being observed.
The echelle-grating spectrometer is a species of grating spectrometer, wherein the grating uses a coarsely spaced grating-groove density at a steep grating angle, and wherein operation is typically in the range of the 20th to the 120th order. All of the orders overlap and must be cross-dispersed, in order to present the spectral information without confusion, and the cross-dispersion has been produced both with gratings and with prisms. The initial echelle-grating spectrometers were used with photographic film as a detector, and Videcon TV cameras and multi-tube photomultipliers (with complicated post-spectrometer optics) have been used. It is difficult to observe and decode all of the spectral information presented, due to compactness of the focused format. Therefore, the instruments have been quite large, to obtain as much freedom as possible, for information retrieval.
A problem in imaging the spectral information of any spectrometer is that the dispersion inherently creates an image plane that extends away from the axis of the focusing optic. This off-axis condition creates image aberrations that destroy the ability of the instrument to use the inherent dispersion of the grating (or prism). The most difficult aberration to deal with is astigmatism, which manifests itself as an imaged vertical line in the focal plane, even though originated as a point of light at entrance to the spectrometer. The astigmatism effect increases dramatically as angles increase away from normal incidence upon the optical system. The effect is not too serious in terms of resolution, since the image remains nearly as narrow as the entrance point, but the intensity of the image suffers since the point has been smeared into a line. Optical detectors must be as big as the line is tall, in order to gather and respond to all of the energy. The difficulty with this is that if a detector (such as a photomultiplier) is inherently round, it becomes difficult to measure individual responses to two dispersed wavelengths that are closely adjacent. Solid-state diode arrays have been developed with many tall and narrow detectors in side-by-side array, but these at best provide only 2048 detector elements that are 25-microns wide; and that is not nearly enough to cover the usually desired spectral range with any resolution that is useful. Scanning instruments (monochromaters) are available to scan the spectra across a single detector; but this is time-consuming. And if the desired information is temporal, the time may not be available. Nevertheless, such scanning instruments are precise, albeit mechanically complex and expensive.
There are now available, commercially, two-dimensional arrays that have as many as 2048.times.2048 (about 4 million) individual detectors, within a 1-cm. square. The desire is to provide a spectrometer configuration that will use this enormous number of detector sites simultanteously. It would seem that the cross-dispersed echelle spectrometer would be the desired design. But the problem is again astigmatism. Since the detectors (pixels) are usually nearly square, there is little room to accommodate this classic aberration, which plagues all spectrometers. It is simply not possible or practical to scale down the design to fit the dispersed spectra onto these 1-cm. square array detectors. Mirrors are forced further off-axis, and the astigmatism worsens.
Bilhorn, et al. papers in Applied Spectroscopy (Vol. 41, No. 7, 1987, at pages 1125 to 1135; and Vol. 43, No. 1, 1989, at pages 1 to 11) describe an echelle spectrometer, wherein echelle orders are dispersed along rows of CID (charge-injection) detectors, each containing 388 23-micron-wide elements. The axis of low dispersion corresponds to columns which contain 244 27-micron-tall elements, for an overall photoactive area 6.6 mm.times.8.7 mm. The spectrometer is a single-pass system, from light source to image plane; it employs a prism for cross-dispersion, and an off-axis Schmidt camera to reduce the size of the focal-plane image to match the sensor dimensions. Allowing for the fact that this spectrometer was limited to 94,672 detector elements (with consequently narrow wavelength range), the results are good, but the optical speed is slow, and the device is large, clumsy and expensive.